Barotropic instabilities of the Agulhas Current system and their relation to ring formation

Recent work has shown that different steady retroflection regimes,a viscous and an inertial regime,exist for the Agulhas Current system in an idealized ge- ometry.In this paper,instabilities of these steady flows are considered by solving the linear stability problem numerically.Barotropic instabilities occur as so-called Hopf bifurcations in the viscous regime,with corresponding patterns related to Rossby basin modes.Depending on the value of the reduced gravity parameter, these instabilities may introduce intermonthly to interannual variability into the retroflection region.Finite amplitude development of these instabilities display "ring-like" localized anomaly patterns which travel around the tip of the continent. The results demonstrate that (i)the origin of the frequency of the ring formation is set by the physics of the large scale barotropic instabilities and (ii)the rectifi- cation processes due to these instabilities decrease the degree of retroflection of the mean state.The latter result suggests that the dominant mechanism of the retroflection is captured within the steady balances

On the physics of the Agulhas Current: steady retroflection regimes

From previous model studies,it has become clear that several physical mech- anisms may be at work in the retroflection of the Agulhas current.Here,a sys- tematic study of steady barotropic flows connecting the Indian Ocean and South Atlantic Ocean in several idealized set-ups is performed.By solving directly for the steady circulation with continuation methods,the connection between differ- ent retroflection regimes can be monitored as external conditions,such as the wind forcing,bottom topography as well as parameters,for example the lateral friction and layer depth,are changed.To distinguish the different steady retroflecting flows,an objective measure of the degree of retroflection,a retroflection index R ,is introduced.By monitoring R along a branch of steady solutions,using the horizontal friction as control parameter,several steady retroflecting regimes are found.At large friction there exist stable steady states with viscously dominated retroflection.When friction is decreased,inertial effects become more dominant and eventually unstable steady states with strong retroflection characteristics ex- ist.Within this framework,different results from earlier studies can be reconciled

Stability of the Atlantic overturning circulation: competition between Bering Strait freshwater flux and Agulhas heat and salt sources

In this study we examine the role that is played by interocean fluxes of buoyancy in stabilizing the present-day overturning circulation of the Atlantic Ocean. A 2D model of the Atlantic overturning circulation is used, in which the interocean fluxes of heat and salt (via the Bering Strait, the Drake Passage and via Agulhas Leakage) are represented by sources and sinks. The profiles and amplitudes of these sources are based mainly on the heat and salt fluxes in a high-resolution ocean model (OCCAM). When applying realistic sources and sinks, a circulation is favored that is characterized by major downwelling in the northern hemisphere (NPP circulation), and resembles the present-day Atlantic overturning circulation. The Southern Ocean sources appear to stabilize this circulation, whereas Bering Strait freshwater input tends to destabilize it. Already a small buoyancy input at southerly latitudes is enough to prohibit the existence of a southern sinking circulation (SPP),lea ving the NPP circulation as a unique and stable solution. A large, factor three increase in Bering Strait freshwater import would be necessary to bring the SPP circulation back into existence. Especially the Indian-Atlantic transfer of heat and salt, brought about by Agulhas Leakage,c ontributes considerably to the strength and, in particular, the stability of the northern sinking circulation. According to this model,sh utting off Agulhas Leakage, and consequently the so-called warm water route for North Atlantic Deep Water (NADW) compensation, leads to a reduction of the overturning strength by 10% at most. These results imply that the way in which the NADW renewal takes place has implications for both the strength and stability of the Atlantic overturning circulation,giv ing the discussion about the warm vs. cold water route for NADW compensation dynamical significance. Moreover, when the stabilizing effect of Agulhas Leakage on the overturning disappears, the destabilizing influence of the Bering Strait freshwater input becomes more effective. The system is then close to a regime where the northern and southern overturning circulations coexist as stable solutions. Perturbations in Bering Strait inflow may then easily lead to switches between the two circulation states. These results suggest that the absence of Agulhas Leakage during the last ice-age may have contributed to weakening the glacial overturning circulation in the Atlantic. It may have made the thermohaline circulation vulnerable to variability, caused either by regime switches, or by the excitation of oscillatory modes. The sudden restart of the Atlantic overturning circulation at the beginning of the Holocene may well have been stimulated by the coincident reopening of the Agulhas gap. The presence of Agulhas Leakage may contribute to the relative stability of Holocene climate. Present-day climate may thus be more stable than previously thought

Response of the Atlantic overturning circulation to South Atlantic sources of buoyancy

The heat and salt input from the Indian to Atlantic Oceans by Agulhas Leakage is found to influence the Atlantic overturning circulation in a low-resolution Ocean General Circulation Model. The model used is the Hamburg Large-Scale Geostrophic (LSG) model, which is forced by mixed boundary conditions. Agulhas Leakage is parameterized by sources of heat and salt in the upper South Atlantic Ocean, that extend well into the intermediate layers. It is shown that the models overturning circulation is sensitive to the applied sources of heat and salt. The response of the overturning strength to changes in the source amplitudes is mainly linear, interrupted once by a stepwise change. The South Atlantic buoyancy sources influence the Atlantic overturning strength by modifying the basin-scale meridional density and pressure gradients. The nonlinear, stepwise response is caused by abrupt changes in the convective activity in the northern North Atlantic. Two additional experiments illustrate the adjustment of the overturning circulation upon sudden introduction of heat and salt sources in the South Atlantic. The North Atlantic overturning circulation responds within a few years after the sources are switched on. This is the time it takes for barotropic and baroclinic Kelvin waves to reach the northern North Atlantic. The advection of the anomalies takes 3 decades to reach the northern North Atlantic. The model results give support to the hypothesis that the re-opening of the Agulhas Gap at the end of the last ice-age, as indicated by palaeoclimatological data, may have stimulated the coincident strengthening of the Atlantic overturning circulation

Impact of interbasin exchange on the Atlantic overturning circulation

The thermohaline exchange between the Atlantic and the Southern Ocean is analyzed, using a data set based on WOCE hydrographic data. It is shown that the salt and heat transports brought about by the South Atlantic subtropical gyre play an essential role in the Atlantic heat and salt budgets. It is found that on average the exported North Atlantic Deep Water (NADW) is fresher than the return flows (basically composed ofth ermocline and intermediate water), indicating that the overturning circulation (OC) exports freshwater from the Atlantic. The sensitivity ofth e OC to interbasin fluxes of heat and salt is studied in a 2D model, representing the Atlantic between 60°N and 30°S. The model is forced by mixed boundary conditions at the surface, and by realistic fluxes of heat and salt at its 30°S boundary. The model circulation turns out to be very sensitive to net buoyancy fluxes through the surface. Both net surface cooling and net surface saltening are sources ofp otential energy and impact positively on the circulation strength. The vertical distributions of the lateral fluxes tend to stabilize the strati fication, and, as they extract potential energy from the system, tend to weaken the flow. These results imply that a change in the composition oft he NADW return transports, whether by a change in the ratio thermocline/intermediate water, or by a change in their thermohaline characteristics, might influence the Atlantic OC considerably. It is also shown that the circulation is much more sensitive to changes in the shape ofth e lateral buoyancy flux than to changes in the shape ofth e surface buoyancy flux, as the latter does not explicitly impact on the potential energy of the system. It is concluded that interocean fluxes ofhe at and salt are important for the strength and operation of the Atlantic thermohaline circulation, and should be correctly represented in models that are used for climate sensitivity studies

Relating Agulhas leakage to the Agulhas Current retroflection location

The relation between the Agulhas Current retroflection location and the magnitude of Agulhas leakage, the transport of water from the Indian to the Atlantic Ocean, is investigated in a high-resolution numerical ocean model. Sudden eastward retreats of the Agulhas Current retroflection loop are linearly related to the shedding of Agulhas rings, where larger retreats generate larger rings. Using numerical Lagrangian floats a 37 year time series of the magnitude of Agulhas leakage in the model is constructed. The time series exhibits large amounts of variability, both on weekly and annual time scales. A linear relation is found between the magnitude of Agulhas leakage and the location of the Agulhas Current retroflection, both binned to three month averages. In the relation, a more westward location of the Agulhas Current retroflection corresponds to an increased transport from the Indian Ocean to the Atlantic Ocean. When this relation is used in a linear regression and applied to almost 20 years of altimetry data, it yields a best estimate of the mean magnitude of Agulhas leakage of 13.2 Sv. The early retroflection of 2000, when Agulhas leakage was probably halved, can be identified using the regression

Predictability and environmental drivers of chlorophyll fluctuations vary across different time scales and regions of the North Sea

Phytoplankton concentrations display strong temporal variability at different time scales. Recent advances in automated moorings enable detailed investigation of this variability. In this study, we analyzed phytoplankton fluctuations at four automated mooring stations in the North Sea, which measured phytoplankton abundance (chlorophyll) and several environmental variables at a temporal resolution of 12–30 min for two to nine years. The stations differed in tidal range, water depth and freshwater influence. This allowed comparison of the predictability and environmental drivers of phytoplankton variability across different time scales and geographical regions. We analyzed the time series using wavelet analysis, cross correlations and generalized additive models to quantify the response of chlorophyll fluorescence to various environmental variables (tidal and meteorological variables, salinity, suspended particulate matter, nitrate and sea surface temperature). Hour-to-hour and day-to-day fluctuations in chlorophyll fluorescence were substantial, and mainly driven by sinking and vertical mixing of phytoplankton cells, horizontal transport of different water masses, and non-photochemical quenching of the fluorescence signal. At the macro-tidal stations, these short-term phytoplankton fluctuations were strongly driven by the tides. Along the Dutch coast, variation in salinity associated with the freshwater influence of the river Rhine played an important role, while in the central North Sea variation in weather conditions was a major determinant of phytoplankton variability. At time scales of weeks to months, solar irradiance, nutrient conditions and thermal stratification were the dominant drivers of changes in chlorophyll concentrations. These results show that the dominant drivers of phytoplankton fluctuations differ across marine environments and time scales. Moreover, our findings show that phytoplankton variability on hourly to daily time scales should not be dismissed as environmental noise, but is related to vertical and horizontal particle transport driven by winds and tides. Quantification of these transport processes contributes to an improved predictability of marine phytoplankton concentrations

On the steadiness of separating and meandering currents

The existence of inertial steady currents that separate from a coast and meander afterward is investigated. By integrating the zonal momentum equation over a suitable area, it is shown that retroflecting currents cannot be steady in a reduced gravity or in a barotropic model of the ocean. Even friction cannot negate this conclusion. Previous literature on this subject, notably the discrepancy between several articles by Nof and Pichevin on the unsteadiness of retroflecting currents and steady solutions presented in other papers, is critically discussed. For more general separating current systems, a local analysis of the zonal momentum balance shows that given a coastal current with a specific zonal momentum structure, an inertial, steady, separating current is unlikely, and the only analytical solution provided in the literature is shown to be inconsistent. In a basin-wide view of these separating current systems, a scaling analysis reveals that steady separation is impossible when the interior flow is nondissipative (e.g., linear Sverdrup-like). These findings point to the possibility that a large part of the variability in the world’s oceans is due to the separation process rather than to instability of a free je

Inertially induced connections between subgyres in the South Indian Ocean

A barotropic shallow-water model and continuation techniques are used to investigate steady solutions in an idealized South Indian Ocean basin containing Madagascar. The aim is to study the role of inertia in a possible connection between two subgyres in the South Indian Ocean. By increasing inertial effects in the model, two different circulation regimes are found. In the weakly nonlinear regime, the subtropical gyre presents a recirculation cell in the southwestern basin, with two boundary currents flowing westward from the southern and northern tips of Madagascar toward Africa. In the highly nonlinear regime, the inertial recirculation of the subtropical gyre is found to the east of Madagascar, while the East Madagascar Current overshoots the island’s southern boundary and connects through a southwestward jet with the current off South Africa